Jennifer N.A. Silva et al. BTS 2018;3:420-430 Principles of Vergence and Accommodation Extended reality platforms rely on positional tracking systems to track head mounted display, hand, and instrument positions. Tracking systems are either external (“outside-in”) systems (left) or on-board (“inside-out”) systems (right). Outside-in optical systems use visual and time-of-flight cameras to track the relative angles to triangulate positions and poses of active or passive marker arrays mounted to the headset, controllers, and tools. Inside-out tracking systems track the relative angles, hands, tools, and landmarks to triangulate the position and pose of the headset relative to landmarks, and the position of hands and tools relative to the headset without any external hardware. Jennifer N.A. Silva et al. BTS 2018;3:420-430 2018 The Authors